Catumaxomab belongs to a novel group of engineered monoclonal antibodies termed trifunctional antibodies [4-5], which typically bind two specific epitopes. Catumaxomab binds the target cell specific antigen CD3E and the T cell specific antigen EpCAM (binding associated with the antibody variable Fv domains). The third type of antigen bound by catumaxomab are the Fcγ receptors on cells such as monocytes/macrophages, natural killer cells and dendritic cells (binding via intact antibody constant domain). Binding across all 3 epitopes selectively targets the immune destruction of target cells e.g. specific cancer cells. In this case the antibody is also hybrid, in that it contains peptide chains of different species origin; one heavy chain and one light chain from mouse and the same from rat [5].